There are many procedures associated with the intra oral cavity in which a precise three-dimensional virtual representation of the intra oral cavity can be useful to the dental practitioner.
Such virtual representations (also referred to interchangeably herein as “virtual models”, “computer models”, “3D numerical entities”, and so on) enable the practitioner to study the intra oral cavity of individual patients via a computer system, in a similar manner to the study of the traditional physical plaster model. Furthermore, three-dimensional numerical entities of the intra oral cavity also allow the practitioner to study methods or approaches when dealing with particular dental problems of any given patient, and for the design of physical entities, for example prostheses, dental brackets, aligners and so on, in relation therewith. For example, in prosthodontics, a computer model of a patient's teeth may be manipulated to provide machining data to manufacture a physical model of the intra oral cavity, and/or to design and manufacture a coping and/or a prosthesis, while in orthodontics a computer model of a patient's teeth may be manipulated to enable a dental appliance, including for example orthodontic brackets and/or aligners, to be designed and manufactured, and/or for designing a treatment.
A parameter used in the design and manufacture of a dental prosthesis, such as a crown or bridge, is the finish line, or transition boundary between the prosthesis and the dental preparation, and this needs to be precisely defined in three-dimensions. Obtaining finish line coordinates from a computer virtual model is more efficient and often more accurate than from a plaster cast, and moreover facilitates the production of such a prosthesis, for example via CNC machining, rapid prototyping, or other computerised technologies, if desired.
However, it is often the case that when scanning the intra oral cavity to obtain 3D data of the preparation and finish line, on which the virtual model is based, part of the finish line, and possibly also the shoulder and other parts of the preparation, may be obscured by soft tissues such as the gum that, no longer being supported by the dental surfaces that have been removed, deform to cover at least a part of the finish line on the prepared dental site.
Additionally or alternatively, part or all of the finish line may be obscured by other agents or materials, including, for example, accumulation of one or more of saliva, blood, lubricant used with a dental drill, debris resulting from working the dental site, and so on.
Similar issues may arise when scanning the intra oral cavity to obtain 3D data of the position and orientation of a dental implant in relation to the surrounding portions of the intra-oral cavity, and in addition the corresponding impression abutment (also referred to herein as a scanning body) may partially obscure part of the intra-oral cavity.
Similarly, there are other situations in which a virtual model of a physical item, obtained from scanning the physical item, is partially obscured or incomplete, or in which part of the physical item needs to be subsequently modified after obtaining the virtual model. Such situations would conventionally require a rescanning of the entire physical item, and this may involve significant additional time, inconvenience and, where the physical item is a part of the body of a patient (such as for example the intra oral cavity), this may also involve significant patient discomfort.